Most antennas are capable of efficient operation over only a limited range of frequencies. For efficient energy radiation or reception, conventional antenna designs calls for antenna dimensions that are on the same order as the operating wavelength (λ). Efficient antenna operation requires not only efficient radiation or reception, the antenna must also be matched to the specific source or load for maximum energy transfer.
Antenna match quality is determined by the voltage standing wave ratio (VSWR) of the antenna at each frequency of interest. A perfect match requires a one-to-one ratio at all frequencies.
For broadband applications, there are several types of antennas that can be designed to provide a low VSWR over wide frequency ranges. Some of these antennas are inherently directional such as the conical spiral and log periodic. Others are omni-directional such as the bicone and tapered blade antennas.
Although there are a number of antenna configurations that can provide a low VSWR over a wide band, most have some limitations that make them unacceptable for many applications. For example, a log periodic antenna can easily be designed to provide low VSWR over several frequency octaves. But, the log periodic phase center, the effective radiating point of the antenna, varies with frequency and a log periodic is physically quite large compared to the wavelength of its lowest operating frequency. A bicone antenna or its monopole is capable of providing low VSWR over a wide bandwidth, but occupies a large volume compared to narrow band antennas having the same low end operating frequency. Generally, wideband antennas are difficult to design for low VSWR over more than an octave frequency range and are generally much larger than a wavelength at their lowest usable frequency.
Antenna, whether broadband or not, are not required to have low VSWR; many conventional antenna designs typically exhibit a high VSWR (>3:1). However, a high VSWR adversely affects efficiency, unless some form of compensatory matching network is used. But matching networks create new problems—most matching networks are not broadband and they tend to decrease the power available for transmission. For high power transmitters, matching networks must often be designed with electro-mechanical tuning elements. Such designs are costly and make automation of the matching function must slower than is possible with lower-power solid state tuning elements. In general, impedance matching to achieve a low VSWR is relatively easy for narrow band antennas (<10% of center frequency), but more difficult for wideband antennas (>20% of center frequency).